From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7DEF713F6DFF for ; Mon, 30 Jul 2018 13:26:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3662020870 for ; Mon, 30 Jul 2018 13:26:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3662020870 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731339AbeG3PBk (ORCPT ); Mon, 30 Jul 2018 11:01:40 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:60648 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726570AbeG3PBk (ORCPT ); Mon, 30 Jul 2018 11:01:40 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4D2C24219DBF; Mon, 30 Jul 2018 13:26:39 +0000 (UTC) Received: from redhat.com (ovpn-117-43.ams2.redhat.com [10.36.117.43]) by smtp.corp.redhat.com (Postfix) with SMTP id 4886E2142F20; Mon, 30 Jul 2018 13:26:33 +0000 (UTC) Date: Mon, 30 Jul 2018 16:26:32 +0300 From: "Michael S. Tsirkin" To: Christoph Hellwig Cc: Will Deacon , Anshuman Khandual , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, aik@ozlabs.ru, robh@kernel.org, joe@perches.com, elfring@users.sourceforge.net, david@gibson.dropbear.id.au, jasowang@redhat.com, benh@kernel.crashing.org, mpe@ellerman.id.au, linuxram@us.ibm.com, haren@linux.vnet.ibm.com, paulus@samba.org, srikar@linux.vnet.ibm.com, robin.murphy@arm.com, jean-philippe.brucker@arm.com, marc.zyngier@arm.com Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Message-ID: <20180730155633-mutt-send-email-mst@kernel.org> References: <20180720035941.6844-1-khandual@linux.vnet.ibm.com> <20180727095804.GA25592@arm.com> <20180730093414.GD26245@infradead.org> <20180730125100-mutt-send-email-mst@kernel.org> <20180730111802.GA9830@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180730111802.GA9830@infradead.org> X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Mon, 30 Jul 2018 13:26:39 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Mon, 30 Jul 2018 13:26:39 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'mst@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 30, 2018 at 04:18:02AM -0700, Christoph Hellwig wrote: > On Mon, Jul 30, 2018 at 01:28:03PM +0300, Michael S. Tsirkin wrote: > > Let me reply to the "crappy" part first: > > So virtio devices can run on another CPU or on a PCI bus. Configuration > > can happen over mupltiple transports. There is a discovery protocol to > > figure out where it is. It has some warts but any real system has warts. > > > > So IMHO virtio running on another CPU isn't "legacy virtual crappy > > virtio". virtio devices that actually sit on a PCI bus aren't "sane" > > simply because the DMA is more convoluted on some architectures. > > All of what you said would be true if virtio didn't claim to be > a PCI device. There's nothing virtio claims to be. It's a PV device that uses PCI for its configuration. Configuration is enumerated on the virtual PCI bus. That part of the interface is emulated PCI. Data path is through a PV device enumerated on the virtio bus. > Once it claims to be a PCI device and we also see > real hardware written to the interface I stand to all what I said > above. Real hardware would reuse parts of the interface but by necessity it needs to behave slightly differently on some platforms. However for some platforms (such as x86) a PV virtio driver will by luck work with a PCI device backend without changes. As these platforms and drivers are widely deployed, some people will deploy hardware like that. Should be a non issue as by definition it's transparent to guests. > > With this out of my system: > > I agree these approaches are hacky. I think it is generally better to > > have virtio feature negotiation tell you whether device runs on a CPU or > > not rather than rely on platform specific ways for this. To this end > > there was a recent proposal to rename VIRTIO_F_IO_BARRIER to > > VIRTIO_F_REAL_DEVICE. It got stuck since "real" sounds vague to people, > > e.g. what if it's a VF - is that real or not? But I can see something > > like e.g. VIRTIO_F_PLATFORM_DMA gaining support. > > > > We would then rename virtio_has_iommu_quirk to virtio_has_dma_quirk > > and test VIRTIO_F_PLATFORM_DMA in addition to the IOMMU thing. > > I don't really care about the exact naming, and indeed a device that > sets the flag doesn't have to be a 'real' device - it just has to act > like one. I explained all the issues that this means (at least relating > to DMA) in one of the previous threads. I believe you refer to this: https://lkml.org/lkml/2018/6/7/15 that was a very helpful list outlining the problems we need to solve, thanks a lot for that! > The important bit is that we can specify exact behavior for both > devices that sets the "I'm real!" flag and that ones that don't exactly > in the spec. I would very much like that, yes. > And that very much excludes arch-specific (or > Xen-specific) overrides. We already committed to a xen specific hack but generally I prefer devices that describe how they work instead of platforms magically guessing, yes. However the question people raise is that DMA API is already full of arch-specific tricks the likes of which are outlined in your post linked above. How is this one much worse? -- MST